1. Field of the Invention
The present invention relates to an alkali-soluble polymer compound containing dehydrogenated resin acid as a monomer component in its single molecule; and a transparent photosensitive resin composition containing the alkali-soluble polymer compound as an effective binder matrix material, which is capable of (1) quickly photo-curing a transparent thin film used in a liquid crystal display, in the form of pattern or entire coating, (2) preventing generation of scum during development, and (3) causing a fewer change in the pattern during a firing process, thereby forming a pattern almost having a vertical shape.
2. Description of the Related Art
A photosensitive resin composition is applied to a substrate to form a film, and the entire film is exposed to form an insulating layer or a protective layer. The photosensitive resin composition is also used to produce a pattern by exposing a particular portion of the film to light irradiation using a photomask or the like, and by removing a non-exposed portion through development.
Since the photosensitive resin composition can be polymerized and cured by irradiating light, it is used for photo-curable ink, a photosensitive printing plate, various photoresists, a color filter photoresist for a liquid crystal display (LCD), a photoresist for a resin black matrix, a transparent photosensitive material, and the like.
Among them, a transparent photosensitive resin composition, which covers a column spacer, an overcoat, a passivation film, refers to a composition in a liquid state comprising an alkali-soluble resin, a polymerizable compound having ethylenically unsaturated bonds, a photopolymerization initiator, and a solvent. This composition usually does not use a coloring agent such as a pigment.
As the use of LCDs is becoming upgraded and diversified, LCDs are manufactured to be used for TVs, monitors, or the like, as well as for the conventional notebook computers, mobile devices, or the like. Demand for a material that quickly reacts to light and has mechanically excellent physical properties has been increasing to improve productivity and durability of LCDs. In forming a pattern through a photo-lithography or an insulated protective film through full surface exposure, the quality of quickly reacting to light, namely, photosensitivity, is very important. Also, in forming a pattern, a portion not reacted to light needs to be cleansed to prevent a liquid crystal layer from being contaminated or prevent a problem in a subsequent process. In addition, in order to allow LCDs to exhibit the original performance without being damaged by an external impact, mechanical physical properties of the column spacer patterns serving as a support must be excellent. These requirements can be met by increasing an upper area of the patterns.
The alkali-soluble resin is a dominant component that functions as a binder matrix in the photosensitive resin composition. The structure of the alkali-soluble resin must be optimized such that a portion, of the alkali-soluble resin, not reacted to light can be quickly cleansed by an alkaline solution, the alkali-soluble resin can quickly react to light, and a change of the alkali-soluble resin during a firing process in which heat is applied can be minimized. It is particularly important in forming a column spacer on an overcoat thin film that a portion of the alkali-soluble resin not reacted to light needs to be completely removed by an alkaline solution. Since the overcoat is an organic thin film generally formed by crosslinking a resin composition by light or heat, polymer in a column spacer has higher affinity with a crosslinking agent, compared with indium-tin oxide (ITO), an inorganic thin film. Thus, a column spacer resin composition not reacted to light may not be completely cleansed by an alkaline solution but remain in the form of small grains on the overcoat. When a liquid crystal alignment is induced by performing a rubbing or mechanical rubbing process on an alignment film, the remaining grains may be detected as debris to cause a scratch. Also, when crosslinking reaction is quickly done owing to good light reactivity, the crosslinked polymer has a structure with high density, reducing a contaminant source of liquid crystal, a residual film ratio of patterns is increased and other raw materials can be widely used in preparing the composition. The reduction in deformation caused by heat in the firing process is closely connected with apparent strength of the patterns. In the column spacers, before and after the firing process, generally, upper portions thereof undergo deformation, as well as a reduction in the thickness thereof. When such deformation is great, the area of the portions of the column spacers supporting a TFT substrate is reduced and resistance of a panel against an external force is weakened. Thus, maintaining a large supporting area by reducing deformation during the firing process is directly related to maintaining the mechanical physical properties of the column spacers.
In order to obtain such characteristics, there is a need to optimize the structure by using various functional monomers through polymerization in making an alkali-soluble resin, and a method for copolymerization by using an appropriate amount of monomers that are excellent in such characteristics is desirable. The desired effect can be obtained by adjusting a molecular weight and an acid value of the polymers thus obtained, without using an additive.
Transparent photosensitive resin compositions known up to date use polymers obtained by adding various functional monomers to a copolymer of methyl methacrylic acid containing carboxylic acid allowing for cleansing in alkali and benzyl methacrylate providing an adhesive property to a pattern. For example, Korean Laid Open Publication No. 2001-0018075 discloses a method for strengthening chemical bonds of a portion crosslinked by light by adding a self-curable structure to the conventional polymer to improve the difference in solubility, thus increasing resolving power and reducing the amount of a crosslinkable compound. However, with this method, the cleansing characteristics of the portion not exposed to light cannot be improved, so the liquid crystal contamination and the problems which may arise in the rubbing process in the liquid crystal alignment are not resolved. In order to minimize such shortcomings, an additive containing an organic acid such as a citric acid, or the like, may be used, but its use is limited because such an additive is generally costly, has low solubility, and is likely to be a contaminant source of liquid crystal.
Thus, development of a polymer which is capable of providing a desired effect and minimizing the accompanying shortcomings is required.